Traditionally, flights of stairs have been constructed using two or more parallel notched stringers extending between an upper and lower surface. The builder determines the total rise of the flight of stairs, the rise needed for each tread and the number of treads. The stringers are marked with the aid of a framing square. Notches in each stringer are cut to facilitate the attachment of horizontal tread and vertical riser members, and the top and bottom of each stringer are cut at the appropriate angles. The stringers are attached in place, such as to the perimeter joist of a deck. The treads and riser members are then attached to, and extend between, the notches in the stringers.
Because notching a stringer reduces the physical strength of the lumber used for a stringer, it is no longer a “rated” member for bearing strength. Consequently, the member cannot be engineered because the load cannot be calculated except by a certified lumber rating professional. Also, the lumber used to make notched stringers must be significantly larger than would be required if the stringers were un-notched, or more of them must be used, to compensate for the weakness. Such construction increases the cost of stair construction.
Additionally, because of the varying floor space available for a given flight of stairs and because of the varying vertical distance between the lower and upper levels of buildings or decks, notched stringers cannot be mass-produced. Instead, they are generally custom-made at the construction site, adding further to the cost. Thus, the cost of building a flight of stairs has remained inordinately high. Practical problems have also been encountered in some cases because of inaccurate calculations, which may result in a riser having a different vertical dimension than that of the remaining risers or, in other cases, because of inaccurate notching of one stringer in a set of stringers. If the notches on a set of stringers are not precisely matched, the tread and riser members will not align properly and the resulting stair system may be weak and shoddy-looking.
More recently, brackets have been used to attach wooden stair treads to stringers (along lines made using a framing square). Even with brackets, however, determining the correct geometry for a particular flight of stairs, and then using the geometry to construct the stairs, remains difficult and leaves little room for error. Also, even with existing brackets, the span between stringers is limited by the structural ability of the wooden stair tread material.
The use of engineered materials, also known as man-made or composite materials in construction, such as for exterior decking, is becoming more common, especially because of their low maintenance requirements. However, a disadvantage of man-made materials for stair treads is that they are not strong enough to span the same distance as a natural wood tread and need support every 8 to 12 inches along the tread rather than every 16 to 24 inches for wood. Because the span between stringers is limited by the structural ability of the man-made stair tread material more stringers are required to support the weak tread material, again adding to the complexity of construction and its costs of time and materials. And, if the stringers are notched, they are weakened, as described above, which is another reason that more stringers would be required. Thus, even the use of conventional brackets does not affect the necessity for the additional stringers.